Screening machine

ABSTRACT

A screening machine is disclosed for separating solid particles in liquid slurry stock wherein a substantially closed surface rotor opposite a screen forms therewith a screening zone and is utilized as a pumping means for recirculating to the inlet to the screening zone rejected stock which has passed through the screening zone. Detrimental thickening of such rejected stock is thereby avoided and the recovery of acceptable particles through the screen is increased.

limited States hatent [191 Bolton, EH et at.

14 1 Apr. 10, 1973 SCREENING MACHHNE Inventors: Joseph A. Bolton, in, North Attleboro; Peter Edmund LeBlanc, Walpole, both of Mass.

Bird Machine Company, South Walpole, Mass.

Assignee: linc.

Filed: Dec. 16, 1970 App1.No.: 98,738

U.S. Cl. ..209/240, 209/273, 209/306, 210/304, 210/415 Int. Cl. ..B07b 1/20 Field of Search ..209/273, 283, 305, 209/306, 380, 242, 300, 358, 270, 240; 210/304,:115, 194

[56] References Cited UNITED STATES PATENTS 3,458,038 7/1969 Young ..209/273 x 2,337,113 12 1943 Knight ..209/306 )1 3,400,820 9/1968 Nelson ..209/273 3,410,409 1 1/1968 Rittenhouse et a1 ..209/273 X Primary ExaminerTim R. Miles Assistant Examiner-William Cuchlinski, Jr. Att0rneyEdgar H. Kent [5 7 ABSTRACT A screening machine is disclosed for separating solid particles in liquid slurry stock wherein a substantially closed surface rotor opposite a screen forms therewith a screening zone and is utilized as a pumping means for recirculating to the inlet to the screening zone rejected stock which has passed through the screening zone. Detrimental thickening of such rejected stock is thereby avoided and the recovery of acceptable particles through the screen is increased.

8 Claims, 4 Drawing Figures PATENTEDAPR 1 01915 SHEET 1 0r 3 LEE 2:: LEE

PATENTEDAPRIOISB I 3,726,401

sum 3 [IF 3 "IIFIG.'4

SCREENING MACHINE This invention relates to machines for screening liquid slurry stock of solid materials such as fibrous paper-making pulp, and particularly to such machines wherein a screenand a substantially impervious rotor define between them a screening passage for separating desired from undesired solid material of the slurry.

Many screening machines utilize a substantially impervious rotor opposite a screen plate to provide a screening zone, the rotor usually having some kind of spaced protuberances thereon to assist the screening operation by creating positive pulses to drive the desired material through the screen and negative pulses to clean the mat of unscreened particles off the screen. Such a screening machine, to which the present invention is applicable is, for example, disclosed in U.S. Pat. No. 3,363,759, issued Jan. 16, 1968.

In operating machines of the type referred to, it has been found advantageous in many uses, such as applications to fibrous screening requiring sharp separation between acceptable fibers and unacceptable fibrous or other material, to maintain the rate of withdrawal from the rejects zone, which follows the screening zone in the direction of stock flow, relatively low. However, in operating at a low reject withdrawal rate, the rejects tend to concentrate to an undesirable extent which may, by backing up into the screening area, plug the screen or otherwise interfere with the desired screening action. Dilution in the rejects area will not usually solve this problem sinceit promotes the undesired backing up of the rejects due to the added volume. Also, such dilution may undesirably dilute the accepted stock, upsetting required consistency control.

An object of this invention is therefore to provide a screening machine of the type previously described wherein undesirable concentration of solid material in the rejects area is prevented. Another object is to provide such a machine wherein the means for preventing such concentration do not add significantly to the cost of the machine or of its operation and improve its operation by increasing the amount of acceptable stock recovered.

It has been discovered that the aforesaid objects can be realized by recirculating stock from the rejects area to the screening area and by'effecting such recirculation by means of the rotor. Such recirculation increases slightly but not detrimentally the solids concentration in the screening area, whereas it substantially reduces the solids concentration in the rejects zone so that deleterious reject concentrations are avoided. There is also the additional advantage that the rescreening involved will usually cause more of the acceptable stock to pass through the screen to the accept compartment,

so that the loss of such stock to the rejects is desirably the rotor located near the stock inlet to the machine. Apparently the centrifugal force exerted by the rotor inner wall on the stock so admitted causes the stock to flow axially along the inner wall of the rotor, toward the inlet end and opposite to the screening stock flow in the machine until it is discharged from the rotor into the flow of stock to be screened.

Therefore, in a preferred embodiment of the invention, there is provided in the rotor at a point adjacent the rejects zone, one or more inlets for stock into the rotor, and at a point adjacent the inlet to the screen one or more outlets for stock from the rotor, the stock inlet to the rotor being closer to the axis of its rotation than the stock outlet therefrom so that stock is recirculated through and by means of the rotor to a zone nearer the inlet. Thus, the rejects are continually subject to a rescreening, the whole stock through the screening zone becomes slightly thickened relieving the undesirable thickening in the reject zone, and discharge of rejects in low volume is accomplished without adverse effects on the screening effectiveness of the machine and, indeed, with improved recovery of acceptable stock.

Implementing the rotor to act as a recirculating pump in the manner indicated does not add significantly to the cost of the rotor and additional power required, if any, is also not significant. Other means may be utilized to cause or assist the rotor to pump stock in the desired manner such as angled scoops or straight or helical blades on the inside wall but the arrangement of one or more inlets and outlets as specified is preferred because of its simplicity and the amount of recirculation obtained is easily controlled according to the capacity of the inlets and outlets, the size and rotational speed of the rotor and the stock capacity of the machine.

Utilizing recirculation according to the invention, reject rates of 5 percent or less are successfully maintained with reject withdrawal on a continuous basis and of 3 percent or less, as low as about 1 percent, with reject withdrawal on an intermittent basis. The screening at these low rates is highly effective while avoiding reject, concentrations sufficient to cause plugging problems or otherwise impair the desired operation.

The screen cleaning surface of the rotor may have any form which produces the desired pulsations. Bumped surfaces such as disclosed in U.S. Pat. No. 3,363,759 aforesaid may be considered a preferred form. Similarly effective is a rotor provided with twoor more eccentric lobes extending substantially the full length of the rotor opposite the screen and which may be smooth-surfaced, the eccentricity relative to the axis of rotation providing the desired pulsations. The screen may be of usual form, either round hole or slotted apertures of size and number as required by the particular stock to which the machine is applied. For most paper fiber applications, holes will be of the order of A inch or less diameter and slots of the order of 0.025 inch wide or less by 1 2% inch or more long.

The foregoing and other objects, features and advantages of the invention will be understood in light of the following detailed description taken in connection with the drawings of preferred embodiments wherein:

FIG. 1 is a vertical longitudinal cross section of an embodiment of the invention;

FIG. 2 is a vertical transverse cross section taken on line 2-2 ofFIG. 1;

FIG. 3 is a partially broken away perspective view of a portion of FIG. 1;

FIG. 4 is a view partly in vertical section partly in elevation of a portion of another embodiment.

Referring to FIGS. 1, 2 and 3 the there shown embodiment comprises a pressure casing 10, a generally cylindrical screen 12 and a concentric cylindrical screen-cleaning rotor 14. The term screen-cleaning is meant to refer here to a device operable to cause periodically a negative flow, against the accepts direction of flow, of oversize rejected solids at the screen plate surface. By this action a screen-cleaning device is operable to dislodge over-size particles and enhance flow of acceptable size particles through the screen plate.'

The casing with the screen plate fitted therein defines a supply channel 16 connected to inlet spout 18, an accepts zone 20 defined on the outside of the screen plate 12, connected to accepts spout 22, and a rejects zone indicated as gutter 24 connected to rejects outlet pipe 26, thence to intermittently operable valve 28 operated by timer 30.

Between the screen 12 and surface of rotor 14 is defined screening zone 32. The screen as shown is of the slotted plate type, having a multiplicity of elongated narrow slots 12a therein.

The rotor 14 is hollow and provided with an internal web 34 near the reject end of the machine. To this web is connected shaft 36 which supports rotor 14 within the screen in cantilever fashion. Shaft 36 passes through seal 38 to drive motor 40.

The rotor 14 of this embodiment is of lobed form, having its effective surface directed toward the inlet side of the screen plate. The lobes comprise eccentric portions of the rotor extending throughout the length thereof, which corresponds to the effective length of the screen. In the embodiment shown the rotor is of uniform cross section from end to end. Referring to FIG. 2 the center of rotation of the rotor 14 is at A; The lobes in this embodiment comprise two semi-cylindrical surface formations, formed about centers B and C, which are spaced equally but in opposite directions from center of rotation A. Thus the two surface formations are eccentric with respect to center A, having low points L and high points H, the latter comprising the lobes that have been referred to. The rotor is so shaped, spaced from the screen plate and driven, that for the 1 stock of the relatively constant consistency maintained by the machine, it assists all of the fluid reaching the screen plate to flow therethrough, leaving behind only solids larger than the apertures.

As the lobe portions of the rotor 14 which are closest to the screen rotate past slots 12a, 21 positive outward pulse is created which advances through the slots, carrying with it components which are smaller than the slots while solids larger than the slots are restrained. With these portions suitably close to the screen the flow assisting forces of the rotor acting on the fluid are significant.

The screen cleaning effect of the rotor is achieved just after the rotor surface portions closest to the screen pass the slots, which creates a negative inward pressure pulse producing a back flow tendency sufficient to unseat oversize debris particles from the slots and draw them toward the rotor. The debris particles are then affected by the feeding movement of the stock (which has an axial component, progressing from the supply to the various slots), this tending to move the debris in the direction of the rejects zone and ultimately into reject gutter 24.

Periodically the timer 30 operates reject valve 28, which discharges the reject gutter and draws a sudden burst of fluid from the screening zone 32, purging the system of debris.

In accordance with the invention, means are provided for pumping stock from the region of the reject zone or gutter 24 to the region of the inlet to screening zone 32. As shown, these means comprise openings 42 through web 34 into the base of the rotor spaced from the inner wall of the rotor. Although the open end of the rotor is exposed to the-stock inlet flow into the open end of the screen indicated by the flow arrows thereon, once the rotor has initially filled with stock, flow takes place therein in reverse direction to the screening flow of stock. This is due to the pumping action of the rotor which draws stock through openings 42, accelerates it by centrifugal force toward the inner wall of the rotor, thereby causing axial flow along the rotor wall and out the open end of the rotor at its edge, as indicated by the flow lines associated with the openings and the rotor. Such flow is caused by the spacing of openings 42 from the inner wall of therotor so that they are closer to the axis of rotation than the outlet which is over the inlet end edge of the rotor wall.

As an alternate or supplement to the open end of the rotor as the outlet for stock therefrom, openings may be provided through the side wall of the rotor at the inlet to the screening zone, such as openings 44 indicated in dotted lines in FIG. 3. With such sidewall openings of sufficient area, the top of the rotor may be closed if desired. The timer controlled valve 28 may be dispensed with and the rejects discharged on a continuous basis, although continuous discharge may involve a somewhat higher reject rate. The screen may alternatively have round holes which may be frusto-conical with the larger end toward the accepts side, or may be of other suitable construction.

The alternative embodiment of FIG. 4 has a casing 50 generall similar to the casing 10 of the FIGS. 1 to 3 embodiment, having a screen 52, which may be round hole, slotted or other type, fixedly mounted in the easing spaced fromthe casing wall to form an accepts compartment 54 (discharge spout therefrom not shown). The rotor, designated generally 56, has in this instance an external surface provided with spaced elevations in the form of bumps 58 such as disclosed in U.S. Pat. No. 3,363,759 aforesaid, and is mounted concentrically within screen 52 by a web 60 fastened to a drive shaft 62 projecting from a supporting pedestal 64. The bumped rotor surface and the screen define between them a screening zone 66, with inlet at one end and outlet at the other end as indicated by the flow arrows, the outlet discharging to rejects zone or gutter 68 from which reject withdrawal may be continuous, or may be intermittent by use of a timer controlled valve such as shown in FIG. 1 (not shown in FIG. 4).

The screening operation of this embodiment is essentially the same as the other, the bumps and the depressions between them respectively creating positive screening and negative screen-cleaning pressure pulses. The rotor inlet for stock from the reject zone is provided by a ring 70 projecting inwardly from the reject end of the rotor toward pedestal 64, the space between the end of ring 70 and pedestal 64 being the inlet to the rotor for stock from rejects zone 68. The outlet for stock from the rotor, which is more remote from the rotor axis than the inlet to the rotor, may be openings 72 in the side wall of the rotor, which are indicated in full lines in FIG. 4, or openings 74 in top web 60 thereof near its outer edge, as indicated by dotted lines in FIG. 4, or both.

Other possible embodiments will occur, such as two of the FIGS. 1 to 3 or FIG. 4 embodiment units placed end to end, with their inlet ends together, as in FIG. 20 of aforesaid US. Pat. No. 3,363,759. In such case the feed to both units would enter between them and the screening flow and opposite rejects flow from the rejects zones at opposite ends of the two screens would be as indicated in the Figures hereof.

It should be understood that the relative amount of stock recirculation through the rotor is a substantial variable. Where it is desired to be able to adjust such recirculation according to conditions, adjustable restriction plates may be provided for the inlet and/or outlet openings of the rotor.

It is realized that the rotational energy applied to the rotor could be utilized to operate a separate pump, e.g., a centrifugal pump, which would recirculate stock from the rejects zone to the screening zone through pipes external to the rotor and screen. Since such an alternative would involve the extra expense of providing and maintaining the separate pumping equipment and piping involved, it is relatively unattractive.

We claim:

1. A screening machine for separating solid particles in liquid slurry stock containing the same which comprises:

a tubular screen;

a rotor mounted within the screen coaxially thereof having a substantially closed surface opposite the inner screen surface and rotatable about the screen axis to define with said screen surface a stock screening zone having an inlet and an outlet;

an accepts compartment opposite the outer surface of said screen;

a stock inlet for supplying stock to be screened to said screening zone inlet; a rejects outlet for removing from the machine stock passing through said screening zone outlet; and

pumping means-operable by the rotational force applied to said rotor to recirculate stock from the vicinity of said screening zone outlet throughthe interior of said rotor to the vicinity of said screening zone inlet.

2. A screening machine according to claim 1 wherein said pumping means comprises a rotor inlet for admitting stock into said rotor in the vicinity of said screening zone outlet and a rotor outlet for discharging stock from said rotor to the vicinity of said screening zone inlet, said rotor inlet being closer to the axis of rotation of said rotor than said rotor outlet.

3. A screening machine according to claim 2 wherein said rotor inlet comprises at least one opening through an end wall of said rotor adjacent said rejects zone.

4, A screening machine according to claim 2 wherein said rotor outlet comprises at least one opening through an end wall of said rotor ad acent said screening zone inlet.

5. A screening machine according to claim 2 wherein said rotor outlet comprises at least one opening through the side wall of said rotor adjacent said screening zone inlet.

6. A screening machine according to claim 1 wherein the surface of said rotor facing said screen comprises lobes eccentric to the axis of rotation.

7. A screening machine according to claim 1 wherein the surface of said rotor facing said screen comprises a plurality of spaced bumps.

8. A screening machine according to claim 1 which includes time controlled means for intermittently removing stock from said rejects outlet. 

1. A screening machine for separating solid particles in liquid slurry stock containing the same which comprises: a tubular screen; a rotor mounted within the screen coaxially thereof having a substantially closed surface opposite the inner screen surface and rotatable about the screen axis to define with said screen surface a stock screening zone having an inlet and an outlet; an accepts compartment opposite the outer surface of said screen; a stock inlet for supplying stock to be screened to said screening zone inlet; a rejects outlet for removing from the machine stock passing through said screening zone outlet; and pumping means operable by the rotational force applied to said rotor to recirculate stock from the vicinity of said screening zone outlet through the interior of said rotor to the vicinity of said screening zone inlet.
 2. A screening machine according to claim 1 wherein said pumping means comprises a rotor inlet for admitting stock into said rotor in the vicinity of said screening zone outlet and a rotor outlet for discharging stock from said rotor to the vicinity of said screening zone inlet, said rotor inlet being closer to the axis of rotation of said rotor than said rotor outlet.
 3. A screening machine according to claim 2 wherein said rotor inlet comprises at least one opening through an end wall of said rotor adjacent said rejects zone.
 4. A screening machine according to claim 2 wherein said rotor outlet comprises at least one opening through an end wall of said rotor adjacent said screening zone inlet.
 5. A screening machine according to claim 2 wherein said rotor outlet comprises at least one opening through the side wall of said rotor adjacent said screening zone inlet.
 6. A screening machine according to claim 1 wherein the surface of said rotor facing said screen comprises lobes eccentric to the axis of rotation.
 7. A screening machine according to claim 1 wherein the surface of said rotor facing said screen comprises a plurality of spaced bumps.
 8. A screening machine according to claim 1 which includes time controlled means for intermittently removing stock from said rejects outlet. 